Video card upgrade assurance?

We just bought a new desktop computer with onboard video. I am wanting to put in a new video card that can handle all games on the market reasonably well. At a decent price what top 5 would you recommend? Here are my specs windows 7 64bit, intel i3 550 @ 3.2ghz, 6.0GB ram, my computer model is lenovo Model: K320-30192GU, link to my system is here http://www.bestbuy.com/site/Lenovo+-+Ideacentre+Desktop+/+Intel%26%23174;+Core%26%23153;+i3+Processor+/+6GB+Memory+/+1TB+Hard+Drive/1339343.p?id=1218251723156&skuId=1339343 . I understand that to do this I am going to have to upgrade my powersupply also, so I am prepared for this as I have a corsair 800w i believe it is. any help would be awesome. also we only have one pcie16x slot